Aircraft powered by jet engines commonly comprise bleed air systems that extract hot pressurised air from the engines for use in various functions around the aircraft such as de-icing systems, cabin pressurisation systems or various pneumatically actuated systems. The bleed air is usually both high temperature and high pressure and as a result requires careful management so as to avoid the associated hazards. Bleed air systems thus comprise one or more safety systems arranged, for example, to detect leakage of the bleed air. Such leakage represents a serious operational issue and generally requires immediate shutdown of the bleed air system and rectification prior to significant further operation of the aircraft. Therefore, it is important for safety systems that detect leakage of bleed air are accurate and robust.